The invention relates to an indexing gear for timekeeping devices, in particular date and day rings in wrist watches. Indexing gears of this type are known, for example, in date indexing devices for analog watches and are constructed such that the weekday and/or calendar date indication can be changed quickly during the night hours. One example is disclosed in U.S. Pat. No. 3,413,800. Another example of the prior art is shown in assignee's U.S. Pat. No. 4,432,081 corresponding to laid open German application DE-OS No. 3046 569 A1, which provides for manual change of the calendar date with respect to the weekday.
In the case of known indexing gears the shoulder of the indexing wheel and the teeth of the pinion are, in view of the distance between the axes of rotation of these two elements, dimensioned such that a certain play is provided between the shoulder of the indexing wheel and the two teeth adjacent to it during intervals between individual indexing operations. This is to prevent rotation of the indexing wheel, which is normally driven by an hour wheel of the watch or clock such that it completes one full revolution within a period of 24 hours, the indexing wheel being obstructed or blocked by engagement of the pinion tooth. In a corresponding way the teeth of the driven toothed wheel and the teeth of the pinion are, with regard to their shape, also coordinated with each other such that the tooth of the pinion engaging with the driven toothed wheel has a certain play so that indexing is easy and requires only low driving power and so that any jamming between the teeth of the pinion and the teeth of the toothed wheel is avoided.
This known solution is disadvantageous in that the indication rings, i.e., the date ring and the day ring, also have a certain clearance between them with regard to their positions of indication due to the clearance between the pinion and the indexing wheel on the one hand and the pinion and the driven toothed wheel on the other. This means that inaccurate or unstable positioning of the indication means may occur, which is annoying for the user. In order to prevent this the driven toothed wheel with the date or day ring is, in the case of the known indexing gears, locked in position between indexing operations by means of a detent, which engages with a corresponding notch in the toothed wheel. This gear construction is relatively complicated due to the additional detent means required. In addition, the detent means have to be released before any adjustment of the day and/or date indication, which does become necessary from time to time, can be carried out. This results in the setting means being more expensive and more complicated in its construction.
It has already been suggested that the pinion be disposed on a displaceable shaft so that the elements of the indexing gear will interact without any play and a stable date or day indication will be obtained. It has been ascertained, however, that it is difficult with this solution to keep the power necessary for indexing and driving the indexing wheel as low as possible, which is always a requirement of wrist watches in particular.
Proceeding on the basis of the prior art, the object underlying the invention is to improve an indexing gear of the type described above, so that this will work without any play even when its construction is simple and the driving power it requires is low.
The decisive advantage of the indexing gear according to the invention is that the play between indexing wheel and pinion, which was a cause for annoyance in the case of the known construction, will be completely avoided. At the same time, an easy and positive driving of the indexing wheel is still guaranteed when the spring action or bias is kept so small that the desired absence of play is just guaranteed so that the indexing wheel can still be driven with low power, even though the teeth of the pinion abut on the shoulder of the toothed wheel for the entire interval between indexing operations. Since, in this way, the clearance between indexing wheel and pinion is completely dispensed with, the necessary play between the driven toothed wheel and the pinion can be kept so small that a stable date or day indication is guaranteed.
Furthermore, it is possible in development of the invention to coordinate the shape of the teeth for both the pinion and the driven toothed wheel such that the tooth of the pinion, which engages with the toothed wheel in the intervals between indexing operations, is adapted to be held in position between the adjacent teeth of the toothed wheel in a pre-biased condition and without any play. It is thereby possible to utilize the same spring action of the teeth of the pinion, which enables the indexing wheel and the pinion to interact without any play.
It has proven advantageous to have the pinion made from an elastically deformable plastic material which is then preferably selected such that only a very slight friction results between the shoulder of the indexing wheel and the pinion. The teeth of the pinion are preferably slotted in their longitudinal direction since this will guarantee not only that the pinion and the indexing wheel interact without any play but also that the pinion and the driven toothed wheel will interact without play.
According to another advantageous embodiment of the invention additional resilient spring tongues, which project outwards, are provided on the tooth flanks of the pinion teeth. These can also provide for sufficient bias between the indexing wheel and the pinion, on the one hand, and the toothed wheel to be driven and the pinion, on the other, when frictional losses are low.
In development of the invention it is also possible to design the teeth of the pinion as spring wires connected to the pinion's hub and bent to the shape of a loop. These may consist of the same material as the hub of the pinion or of another suitable material and form elastically deformable teeth, which can be slightly flattened at their flanks in order to achieve the desired bias relative to the indexing wheel or to the adjacent teeth of the toothed wheel.